OP solved it, closing

this is due to input mutation

Good example; if I can re-publish the kata I will put that in the description. Thanks!

Your code fails for any single digit input (e.g. n=5). This is because your reduce function tries to take two arguments (x and y, representing the digits) however the input is one digit long and the second argument doesn't have value, therefore it returns the first value as a string. It produces an error from m/10 because it is trying to divide a string.

Your transformation from string to number does not work.
In the array there could also be negative numbers! Thats the '-'... ;-)

I hope that helps.

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Your code returns string (proof -> <class 'str'> should equal 0) when the expected outcome should be an integer. Simply cast the string -> to integer.

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